The firehose file (typically named something like prog_emmc_firehose_8953_ddr.mbn ) is a programmer file used by Qualcomm devices. For the Oppo A3s CPH1803
Search for "Oppo A3s CPH1803 QFIL Firmware" on sites like EvilicaCell to find official packages that include the programmer. download firehose oppo a3s cph1803
Open your chosen flashing tool (e.g., QFIL). download firehose oppo a3s cph1803
These are necessary for your computer to recognise the device in EDL mode. download firehose oppo a3s cph1803